1. Field of the Invention
The present invention relates to a printing device such as an LED printer for printing images on paper sheets, using a recording head which includes a large number of light emitting diodes (LEDs) arranged in a line.
2. Description of the Related Art
Generally, an LED printer has a recording head including a large number of LEDs arranged in a line and is used to print an image corresponding to an image signal on a paper sheet. This kind of printer is utilized as a small printer incorporated in a facsimile apparatus, for example. In such an LED printer, the LEDs are selectively activated so as to emit light in accordance with an image signal, whereby the electrostatic image corresponding to the image signal is formed on a charged photoconductive drum. The electrostatic image thus formed is developed using a toner, and the toner image is transferred onto a paper sheet and fixed, thus printing the image corresponding to the image signal on the paper sheet.
In CCITT (International Telegraph and Telephone Consultative Committee), the recording density in the horizontal scanning direction (the direction in which the LEDs are arranged) of a facsimile apparatus (G3 type) is determined as 8 dots/mm, and the recording density in the vertical scanning direction (i.e. the direction in which the photoconductive drum is rotated) is standardized as three values, 3.85 lines/mm, 7.7 lines/mm, and 15.4 lines/mm.
Considering the case where dots having a size according to a recording density of 8 dots/mm in the horizontal scanning direction are printed, when the recording density in the vertical scanning direction is 7.7 lines/mm, printing can be successfully performed as shown in FIG. 1B. When the recording density in the vertical scanning direction is 3.85 lines/mm, printing can also be successfully performed as shown in FIG. 1A, by printing the data corresponding to one line of dots twice.
However, when the recording density in the vertical scanning direction is 15.4 lines/mm, the dots overlap each other, as shown in FIG. 1C, thus degrading the quality of the image, since the recording density in the vertical scanning direction is higher than that in the main-scanning direction. If the size of the dots is decreased in accordance with a recording density of 15.4 lines/mm, the spaces between the dots arranged in the horizontal scanning direction are increased, again resulting in degraded image quality.
Thus, a conventional electrographic printing device has the drawback that when the recording density in the vertical scanning direction is higher than that of the horizontal scanning direction (e.g. 8 dots/mm.times.15.4 lines/mm), the dots overlap each other, decreasing the resolution and image quality.